Field of the Invention
The present invention relates to a method for commanding a multi-axis robot, as well as a robot for implementing such a command method.
Brief Description of the Related Art
It is known to command a multi-axis robot by providing the variable steering drives of its electric motors with command signals generated from a computation unit that determines the trajectory of the robot. The motor of each axis of the robot is equipped with an electromagnetic brake actuated by the computation unit after the controlled stop of the movement on its programmed trajectory, so as to lock the movements of the robot, in order to guarantee the safety of the users and equipment. Once the robot is stopped, the computation unit cuts the motor torque.
It is known from US-A-2002/0084766 to equip the robot with variable drives that control the supply of electricity for the phases of each motor. Each variable drive includes a bridge circuit, which comprises an upper branch equipped with three switchable switches, each connected to a positive bus and to one of the phases of the motor, as well as a lower branch equipped with three additional switchable switches each connected to a negative bus and to one of the phases of the motor.
To allow the emergency stop, the variable drives of US-A-2002/0084766 incorporate a safe torque off function, preventing the switches from switching when it is activated. This function is activated when the command of all of the switches of the two branches of the bridge circuit of a variable drive is neutralized. When a variable drive cuts the power supply of the switches of both branches of the bridge at the same time, this creates a redundancy that makes it possible to improve safety, since the torque will be cut even if a fault occurs resulting in erroneously powering one of the branches of the bridge.
Furthermore, the variable drives make it possible to implement an electric braking function, performed by short-circuiting the phases of the motor that one wishes to brake, i.e., by closing all of the switches of one of the branches of the bridge. The electric braking function is useful to move a moving part of the robot subject to gravity manually in complete safety, since the braking limits the acceleration of the moving part when it is driven downward by its own weight.
When the emergency stop is activated and the user needs to release a brake to move a moving part of the robot, there is then a risk of rapid and uncontrolled movement of that part under the effect of gravity, which is dangerous.
It is necessary to deactivate the torque off function on one of the branches of the bridge to allow activation of the electric braking, so as to reactivate the command switches to close them. This is detrimental to the safety level, since the redundancy introduced by the safe torque off function is then limited, since the switches of one of the branches are no longer neutralized. Standard ISO 10218-1 relative to industrial robots requires a safety level SIL 2 and a redundancy on the emergency stop function. These safety requirements are then no longer met.